As an experiment, could you try moving one of these messages to the Inbox in Local Folders, and then mark it as Not Junk?

My theory is that some outside agency is doing this, and your moving a message to Local Folders takes it beyond the reach of email servers. If it stays put, then we have a strong indication that Thunderbird isn't doing this.

Zenos said

As an experiment, could you try moving one of these messages to the Inbox in Local Folders, and then mark it as Not Junk? My theory is that some outside agency is doing this, and your moving a message to Local Folders takes it beyond the reach of email servers. If it stays put, then we have a strong indication that Thunderbird isn't doing this.

Interestingly, moving the email to the inbox in Local Folders did exactly that, it could be marked as "Not Junk" and remained that way. That doesn't resolve the problem though. I still have all email from this one friend arriving as junk. When my ISP "marks" email as junk, it goes into a Folder named "Bulk Mail" but this is going into my Inbox. Even if I turn off adaptive junk mail controls, his emails still go into Inbox marked as junk.

It's not the end of the world if I can't fix this, I still get his emails in my inbox.

Also, and perhaps a different problem, if I move it back to the original inbox, it appears to randomly grab the content of another email in the inbox as its own, losing the original content entirely and irrevocably.

Are you using Yahoo mail? They apparently think they know better than their customers and decide for you what is junk and no amount of trying can either opt out of their silly filter, or make it do as you want.

Matt said

Are you using Yahoo mail? They apparently think they know better than their customers and decide for you what is junk and no amount of trying can either opt out of their silly filter, or make it do as you want.

I do use Yahoo, or more correctly, Rogers uses Yahoo as their mail provider. But Yahoo isn't tagging the emails in question as far as I can tell. With Yahoo, emails are put into the bulk mail folder, these are still going to my inbox and Thunderbird is telling me it thinks they're junk.

It's not the end of the world, I'm still seeing all his mail. I just have to click "Not Junk" to see images. If I go back after clicking "Not Junk" previously, I have to again because it's been re-marked as junk.

The "issue" is that Thunderbird isn't following the rule I've set that all mail from his address should not be marked Junk and even after a manually mark it "Not Junk", it reverts.

I think Zonos has it right, because it can be permanently marked not junk in a different folder, it's tied to the server. I'll mark his answer as a "solution" even though it doesn't actually solve the issue. **Shrug**

You might think it is not Yahoo, but you would be wrong. This happens to Yahoo customers all the time.